In recent years Machine-Type Communication (MTC) has attracted increasing interest from the mobile community. An example of this is the recently begun “Study Item on Cellular System Support for Ultra Low Complexity and Low Throughput Internet of Things (FS_IoT_LC)” in the GERAN working group of the 3rd-Generation Partnership Project, on providing cellular access for devices falling into the category of Internet of Things (IoT). This study item is described in the 3GGP work item description GP-140421, “New Study Item on Cellular System Support for Ultra Low Complexity and Low Throughput Internet of Things (FS_IoT_LC) (revision of GP-140418)”, source VODAFONE Group Plc, available at http://http://www.3gpp.org/ftp/tsg_geran/TSG_GERAN/GERAN_62_Valencia/docs/GP-140421.zip. The focus of this study is to develop a solution that provides a high system capacity and improved coverage for equipment with low requirements on data rate and latency.
One of the proposals for a candidate solution in this study is the so-called Narrow Band Hybrid Modulation, as described in the 3GPP report GP-140583, “Narrow Band Hybrid Modulation for Cellular Iot,” source Nokia Networks, available at http://www.3gpp.org/ftp/tsg_geran/TSG_GERAN/GERAN_63_Ljubljana/Docs/GP-140583.zip. The technique described in this report was subsequently renamed “Narrowband GSM,” or “N-GSM,” when discussed in the 3GPP Technical Report, “Cellular System Support for Ulta Low Complexity and Low Throughput Internet of Things,” 3GPP TR 45.820, v. 1.3.1, 12 May 2015, available at http://www.3gpp.org/DynaReport/45820.htm. In the discussion that follows, the terms “N-GSM” and “Narrow Band Hybrid Modulation” will be used interchangeably.
Narrow Band Hybrid Modulation or N-GSM, as described in the GP-140583 report, reuses the legacy GSM transmitter architecture, but with the addition of a pre-coding module inserted before the modulator. This is shown in FIG. 1, which illustrates pre-coding module 110 and a conventional GSM/GPRS modulator 120. Data bits b(i) are encoded by pre-coding module 110, which maps each data bit b(i) or, a group of data bits, to a multi-symbol pattern, which is then used as the input to GSM/GPRS modulator 120, which modulates a carrier frequency with the multi-symbol pattern, using Gaussian Minimum-Shift Keying (GMSK) or Phase-Shift Keying (PSK) modulation.
In its simplest realization, pre-coder module 110 maps each data bit onto an eight-bit bit-pattern that is sent to the modulator 120. When a GMSK modulation and coding scheme (MCS) is used, for example, the pre-coder module can map an input value of 0 to the repeated bit pattern “0000000,” which will result in a narrow band signal, a tone, located at an offset around 68 kHz above the center frequency of the occupied radio channel. An input value of 1 can be mapped to the alternating bit pattern “10101010,” which will result in a tone located at an offset around 68 kilohertz (kHz) below the center frequency of the occupied radio channel.
The GP-140583 report discussed above provides additional details for various precoding options, such that one to three data bits can be mapped to 8-symbol sequences supplied to a GSM/GPRS modulator, where seven 8-symbol sequences can be transmitted within a conventional GSM/GPRS burst. As shown in the report, the simple mapping example provided above can be extended to 8PSK modulation, in which case each possible combination of three input data bits is mapped to one of 8 different tones, each of which, because of the length of the multi-symbol pattern supplied to modulator 120, occupies only a small part of the bandwidth of the 200-kHz GSM channel at any given time.
At the receiver side, all a device must do to demodulate a N-GSM signal formed in this matter is to detect which tones in the targeted channel were received, e.g., using a simple Fast Fourier Transform. In addition, information might be carried in the phase/amplitude of the signal, but this example is limited to information transmitted only by different tones. The receiver can then map the detected tones to data bits. This new concept is claimed to be of reasonable computational complexity and to provide the level of coverage and the bit rates required by the FS_IoT_LC study.
The N-GSM solution described above, however, will suffer from low spectral efficiency, leading to reduced system capacity since the technique during transmission only occupy parts of the available GSM/GPRS channel bandwidth. Compared to the GSM/GPRS legacy symbol rate of 270 kHz, this new solution will, at any given instant, only occupy fractions of the GSM/GPRS 200-kHz channel bandwidth. Since the study item discussed above has a clear focus on providing good system capacity, this is a drawback in terms of resource utilization.